1. Field of the Invention
This invention relates to a drawing tool for deforming preferably planar sheet metal elements, comprising a drawing die, which is operable by an external force, a blank holder, which is movable in unison with the drawing die, a drawing punch, which is adapted to be forced into the drawing die so as to deform the blank, and a hydraulic element for deriving from the movement of the die and blank and the holder a simultaneous, opposite movement that is imparted to the drawing punch, which hydraulic element comprises a centrally disposed, differential working piston, an annular piston, which surrounds said working piston, and a cylinder housing, which surrounds the annular piston, wherein the annular piston is indirectly operable by the external force and adapted to be supported by the fluid in a cylinder chamber and adapted to be supplied with pressure fluid, and the working piston is adapted to be supplied with pressure fluid at both ends and is adapted to be displaced by the pressure fluid which is displaced out of the cylinder chamber by the annular piston during the drawing operation.
2. Description of the Prior Art
In a known drawing tool of that kind the drawing operation is effected by a drawing movement of the externally actuated drawing die and an oppositely directed drawing movement which is performed by the drawing punch at the same time because the working piston is operated to move opposite to the die and blank holder by the pressure fluid that is displaced by the annular piston. A common cylinder chamber is provided for the annular piston and the working piston. After the drawing operation the drawing tool is reset to its initial position by pressure applied to a differential surface area of the working piston. Such a drawing tool has been disclosed in U.S. Pat. No. 4,796,453, the disclosure of which is incorporated herein by reference. Other prior art is apparent from U.S. Pat. No. 2,609,775 and from Australian Patent Publication No. 132,028.
In a drawing tool of a different kind it is known to change the drawing force and the blank-holding force by means of hydraulic controls in that pressure fluid is drained from the respective cylinder chambers into a reservoir. Those volumes of pressure fluid which are required to be contained in the respective cylinder chambers for the conventional drawing operation can be re-established by a supply of fluid from a pressure accumulator. Because the drawing die cushion is relatively large in that drawing tool, the latter can be used only in large presses (Published German Application No. 3,603,107).